A lithium-ion battery has many merits, such as high energy density per unit weight and per unit volume, long recycle life, required safety and reliability, quick charge/discharge, and so on. Therefore, in recent years, a lithium-ion battery has become a hot topic in research and development of new energy storage technologies, and been popularly applied to fields of high energy and high power. A common lithium-ion secondary battery consists of a material for a positive electrode, a material for a negative electrode, an electrolyte, a separator and a packaging material for battery shell.
The lithium-ion battery having a solid polymer electrolyte (SPE) is so called as a polymer lithium-ion battery. The polymer lithium-ion battery is made up with a positive current collector, a positive electrode film, a solid polymer electrolyte membrane, a negative electrode film and a negative current collector. The shell packaging material of the polymer lithium-ion battery is an aluminum-plastic composite film, and the edge of the shell is sealed by a hot melt process. A polymer membrane doped with a certain proportion of inorganic salt grains, such as SiO2, Al2O3, and LiCF3SO3, is directly used as an electrolyte in the current solid polymer electrolyte membrane. The representative polymer is polyethylene oxide (PEO), and a thickness of this kind of electrolyte is generally more than 100 μm, wherein the lithium-ion of the electrolyte can move in the molecular chain. Meanwhile, when a copper foil is used as the current collector metal, for example, copper ions resulting from oxidation of the copper foil may also pass through the solid polymer electrolyte membrane easily. Once the battery is over-discharging, it is likely to cause oxidation of the copper foil so that a large number of copper ions generated accordingly would smoothly move to the surface of the positive electrode material. Besides, this kind of solid electrolyte polymer membrane has a main drawback of relatively low ionic conductivity, which is about 10−5 S/cm at room temperature and smaller than that of a liquid electrolytic solution with LiPF6 used as an electrolyte by about two orders of magnitude.
When the lithium-ion battery is used as a power battery, in response to requirements on levels of voltage and capacity, it is common to connect a plurality of single cells in series or in parallel to achieve the requirement on operating voltage and capacity of the electrical facility. When the battery is actually connected to a load, and if the battery management system suffers from deficient management conditions and techniques, some of the single cells might be over-discharging as there exists inherent inconsistency among cells. The over-discharge state would cause the overly de-intercalation of the lithium ions from the negative electrode and oxidation of the negative current collector which is usually made of a metal such as a copper foil. Meanwhile, the metal ions generated from the negative current collector will pass through the separator with physical through pores, which are widely used in lithium-ion batteries, e.g. a separator with physical through pores produced by the Celgard Corporation, the United States, or the UBE Company, Japan, and arrive at the positive electrode. As a result, irreversible battery operation as well as short-circuit between the electrodes might occur. Accordingly, the recycle life and the capacity of the battery would be adversely affected, and meanwhile, a risk in safety would be rendered.
Chinese Patent No. CN2922234 discloses an over-discharge protection circuit able to prevent a power lithium-ion battery from over-discharging, which includes a pair of input and output ports and a battery unit. A discharge control switch is configured to be connected between one of the input and output ports and an electrode of the battery unit. The battery unit consists of multiple lithium-ion rechargeable cells interconnected in series. Furthermore, the protection circuit includes an over-discharge voltage-detecting unit able to compare the voltage of the battery with a set voltage and output a comparison result, and an over-discharge logic control unit able to control the on/off state of the discharge control switch according to the output signals outputted by the over-discharge voltage-detecting unit. When the voltage of any of the cells is below the set discharge limiting voltage, or when the voltage of the battery pack is below the set voltage, the over-discharge protection circuit can automatically open the circuit so as to stop discharging, thereby preventing each of the cells from being damaged by over-charging, and ensuring the battery unit safe discharge.
Chinese Patent No. CN101404406 discloses a protection circuit of a lithium battery. The protection circuit consists of an over-charge control tube, an over-discharge control tube and a protection IC. The battery voltages in the over-charging control tube and the over-discharge control tube are monitored and controlled by the protection IC. The protection IC is a CMOS integrated circuit block, including an over-charge protection circuit, an over-discharge protection circuit and an over-current protection circuit. In the protection IC, a clamping circuit is disposed between the output negative electrode V− and the gate electrode of the MOS transistor coupled to the output negative electrodes V−. When the voltage of the output negative electrode V− changes in a wide range, the clamping circuit bears most of the negative voltage, so as to make the voltage bore by the gate electrode of the MOS transistor be limited to less than −2.5V without affecting the functions of the over-charge protection circuit, the over-discharge protection circuit and the over-current protection circuit.
Chinese Patent No. CN101159375 discloses power supply control and protection circuit and method of a lithium battery. By detecting the voltage of the battery, and controlling the power supply of the battery, this method can prevent the battery from over-discharging. Furthermore, the battery can be turned off by way of software.
In all the aforementioned prior arts, the objective of preventing the lithium-ion battery from over-discharging is achieved by way of external circuit designs without substantially or internally avoiding the damage caused by the excessive discharge of the lithium-ion battery. The high-capacity battery pack or the system of the battery pack often consists of thousands or even tens of thousands of battery cells, so it is difficult to use an external circuit design to protect a single cell. Furthermore, it is hard to guarantee reliability of the circuit, and its management cost is relatively high.